Numerous electrochemical processes are widely used, and examples include electric storage batteries, production of basic raw materials (e.g., aluminum, chlorine, caustic soda, titanium, hydrogen, etc.). More recently, electrochemical devices are also used in medical devices to monitor blood sugar, drugs, or help analyze DNA. Thus, and depending on the type of use, electrochemical devices may considerably vary in size and overall design.
Remarkably, despite the enormous variety of electrochemical devices, the electrodes and their configurations substantially remained the same. Among other factors, the common thread in electrochemical processes and processing is that they take place by the movement of ions or electronically charged species in an electrolyte. Another common feature is the use of electrodes where electrons are transferred in and out of the electrolyte, which can be a liquid, a molten salt, or even a gelled or solid solution. During an electrochemical reaction it should be noted that electrons, the charge carriers, move through electrodes at the speed of light, whereas ions, the carriers of electrons through the electrolyte occur at much lower speeds, millimeters per second at best. Consequently, the rate of reaction is predominantly controlled by the ability of ions to participate in the reactions at the electrode/electrolyte interface. Therefore, the availability of the surface and the concentration of the reactive species at that surface are fundamentally important. The parameters that govern reaction rates at electrode interfaces include the rate that reactants reach the electrode, and the rate at which products diffuse back into the bulk of the electrolyte. These parameters are further compounded by concentration factors of reactants, products, and solute molecules which can slow down the speed at which the reactants make contact with the electrode prior to the fast electron transfer reaction taking place.
The electrolyte layer immediately adjacent to the electrode surface is generally known as the Nernst diffusion layer, and it is assumed that mixing in this layer between and the bulk of the solution is subject to Fick's law of diffusion. Many different equations are based on these considerations and have been employed to explain the kinetics and rates of reaction for various situations. Notably, it is generally accepted that dispersion of the Nernst diffusion layer by shear or turbulent flow will increase reaction rates. In common practice and for most of the electrochemical reactions, this theory holds true.
Reaction rates in most conventional electrochemical reactions (e.g., electrosynthesis, large scale electrolyzers for metal winning, water sterilization, batteries, fuel cells, etc.) are typically controlled by current and limited by mass transfer. Therefore, many electrochemists and engineers attempt to improve electrochemical reaction kinetics by increasing the flow rate of fluid that passes over an electrode surface to thereby increase sheer stress that is thought to increase mass transfer by mixing. This is often achieved by pumping or stirring, oscillating the electrolyte, or by rotating the electrodes to disperse the electrolyte at the electrode interface. These techniques help to at least some degree to remove or displace the products of the reaction and solvent molecules from the reaction surface allowing greater access for reactant ions. Alternatively, or additionally, the surface area of the electrode may be increased. However, numerous difficulties still remain. Among other things, diminishing returns will arise as reactant concentrations decline, and increasing the size of the electrodes will fairly quickly reach practical limitations.
To analyze in more detail the flow characteristics at the electrode to solve problems associated with large electrodes and flow properties at the electrode surface, electrochemists and chemical engineers have historically assumed that the theoretical treatment of flow in pipes by dimensionless numbers analysis is sufficient to characterize the conditions found in most electrolyzers. The boundary layers that impede electron transfer reactions known as the Nernst diffusion layer may be reduced by creating turbulent flow. Figures of merit are expressed as Reynolds numbers greater than 3000 to describe turbulent flow and below 2000 for laminar with 2000 to 3000 considered the critical or transient flow. However, when cell gaps are less than 1 mm the model fails as the boundary layers collapse on each other and flow equivalent to a gravity fed cell with only tens of cm of head would produce a well mixed thin boundary layer surface by laminar flow alone.
Models used to calculate and study the reaction and electrolyte kinetics are referred to as mass transport phenomena. In order to simplify the mathematics of hydrodynamic or mass transport phenomena, use is made of dimensionless numbers (e.g., Reynold's number, Schmidt number etc.) or ratios of parameters that numerically describe the physical properties of a solution without units. For example, Reynolds number is ρvd/μ where ρ is the density, v is the linear velocity, d is the diameter of a pipe, and μ is the kinematic viscosity. This artificial mathematical modeling often works fairly well in certain systems, but fails as the size of the gap between the walls of the system, d, becomes very small as turbulent flow is restricted and laminar flow is predominant.
Therefore, while numerous configurations and methods of electrochemical devices are known in the art, all or almost all of them suffer from one or more disadvantages, especially where the cell gap is relatively small. Consequently, there is still a need to provide improved composition and methods to improve wear resistance in such products.